A series of crude bombs went off in the eastern Indian city of Patna on Sunday killing at least five people, minutes before opposition leader and prime ministerial candidate Narendra Modi addressed a rally.
The rally was the biggest so far by the main opposition Bharatiya Janata Party (BJP) ahead of general elections in 2014.
SIx low intensity explosions were reported in and around Gandhi Maidan, the rally venue named after Mahatma Gandhi in the heart of the city in the northern state of Bihar.
Blasts were reported from within the grounds, outside a cinema hall and the railway station all within a 2-km radius.
Junior home minister R P N Singh said: "These were low intensity bomb blasts. It's extremely unfortunate these blasts have taken place."
Some Indian television channels reported that one person was killed in the blast at the railway station.
There was some panic in once section of the rally venue as television footage showed plumes of smoke rising and people running away.
Some 20 minutes after the blasts, Mr Modi and the senior leadership of the BJP including its president Rajnath Singh took to the stage in a show of strength.
Chants of Modi reveberated as he took to the stage.
He attacked his main opponent Congress vice-president Rahul Gandhi and Mr Nitish Kumar, the chief minister of the state of Bihar and leader of Janata Dal United (JDU), in the latest series of rallies he has addressed in his bid to become the prime minister of the country.
The state of Bihar is important for the BJP, which lost its key long-time ally JDU after Mr Modi was elevated as poll panel chief in June this year.
The JDU, a regional party that is in power in the state, had partnered the BJP for more than 16 years in a powerful alliance both in the state of Bihar and at the federal level.
The party had pulled out of the alliance fearful that it would lose its Muslim support base over the elevation of Mr Modi, who was at the helm during one of the worst Hindu-Muslim riots India has seen.
Sunday's bomb explosions have also brought focus on law and order in the city, which was seen to have improved under Mr Kumar, who held an emergency meet over the blasts.
In July this year, a series of bomb blasts took place at the world famous Mahabodhi Temple where Buddha found enlightenment. Two monks were injured in the attack.
On Sunday's blasts, Congress general secretary Digvijaya Singh tweeted: "What a coincidence blast at Patna Railway on the day of Modi's Rally! Challenge to Nitish Govt to find the culprit!"
The federal government despatched a seven-member team from the National Investigation Agency and National Security Guards to Patna to join the investigations. Incidentally Mr Modi took to the stage in Patna at the same time as Mr Rahul Gandhi addressed a rally in the country's capital city of Delhi.
Just last week Mr Gandhi, whose father and grandmother were both assassinated, said he too may be killed in a similar fashion.